1. Field of the Invention
The present invention relates to a spectrometric method and an apparatus for spectrometry. In particular, it relates to a spectrometric method and an apparatus for spectrometry which use a tunable laser as a spectrometric light source, and a method for spectroscopic analysis and an apparatus for spectroscopic analysis for measuring emitted light emanated from a sample.
Meaning of the term "emitted light" used herein generically includes fluorescence and scattered light resulting from Raman scattering, which are emanated from a sample.
2. Description of the Prior Art
As sample analyzing methods having high sensitivity, spectrometric methods have been widely employed which comprise irradiating a sample with light in infrared region, visible region or ultraviolet region, and measuring, for example, absorption of the light by the sample, or light from the sample attributable to reflection, Rayleigh scattering, Raman scattering or fluorescence. By means of such spectrometric methods, it is possible to perform qualitative analysis such as identification or confirmation of a sample, quantitative analysis such as determination of concentration of a specific ingredient contained in a sample or determination of composition of a mixture, and analysis of electronic state or stereostructure of a molecule. If absorption spectrum of a sample is measured by means of time-resolution, it is also possible to perform analysis of progress of reaction or analysis of molecular structure of an intermediate
In spectrometry, it is required to irradiate a sample with monochromatic light having a specific selected wavelength or to irradiate a sample with monochromatic light while continuously sweeping wavelength thereof. In general, a spectrophotometer is employed which comprises a white-light source and a monochromator in combination to obtain monochromatic light. Sweeping of wavelength of the monochromatic light emerging from an exit slit of the monochromator is effected by rotating a wavelength dispersing element, for example a diffraction grating, which is incorporated in the monochromator.
FIG. 23 is a diagrammatic view of a double-beam spectrophotometer as one form of conventional spectrophotometers. The spectrophotometer is for measuring absorption spectrum of a sample. A light beam emitted from a light source such as a tungsten lamp or deuterium discharge lamp, which is disposed in a light source section 200, passes through an entrance slit S, of a monochromator 210 and impinges upon a diffraction grating G via mirrors M.sub.1, M.sub.2. The light beam is subjected to wavelength dispersion by the diffraction grating G, and then it travels via mirrors M.sub.3, M.sub.4 and forms a spectral image at a position of an exit slit S.sub.2. In consequence, monochromatic light 211 emerges from the exit slit S.sub.2. The monochromatic light 211 emanated from the monochromator 210 enters a first sector mirror 221 which is rotating. The first sector mirror 221 has a light transmitting portion and a light reflecting portion and alternately directs the light beam to an optical path to a sample S and an optical path to a reference sample R. The monochromatic light 211 transmitted through the first sector mirror 221 enters the sample S. The light transmitted through the sample is reflected by a reflecting mirror 223 and then reflected by a second sector mirror 224, which is rotating synchronously with the sector mirror 221 under control of a controlling section 220, to enter a detector 230. On the other hand, the monochromatic light 211 reflected by the first sector mirror 221 is reflected by a reflecting mirror 222. Then, it is transmitted through the reference sample R and through the second sector mirror 224 and detected by the detector 230.
In some spectrometric methods, a tunable laser is used as a light source. As the tunable laser, there have been known a solid-state laser using a crystal of Ti:Al.sub.2 O.sub.3 (titanium-containing sapphire) or the like as a laser medium and a liquid laser using a dye solution or the like as a laser medium As a wavelength selection method for inducing laser oscillation of such a tunable laser at a desired wavelength, for example, there has been employed one which comprises placing a diffraction grating, birefringent plate or the like in a laser resonator containing a laser medium, and allowing oscillation at only a specific wavelength to be induced in the laser resonator by mechanically rotating the diffraction grating or the like to obtain a laser beam having a desired wavelength.
Next, measurements of Raman scattering will be described. When a sample is irradiated with a laser beam having a frequency of .upsilon..sub.0 =c/.lambda. wherein c is the light velocity and .lambda. is the wavelength of light), scattered light resulting from Raman scattering (Raman scattered light) is obtained which has frequency components shifted from the frequency .upsilon..sub.0 of the incident light in an amount of .+-..DELTA..upsilon.. The differences .+-..DELTA..upsilon. between the frequency .upsilon..sub.0 of the incident light and the frequencies of the Raman scattered light are referred to as "Raman shift"s Of Raman lines, one having the frequency (.upsilon..sub.0 -.DELTA..upsilon.) lower than that of the frequency .upsilon..sub.0 of the incident light is referred to as a Stokes line and the other having the frequency (.upsilon..sub.0 +.DELTA..upsilon.) higher than that of the frequency .upsilon..sub.0 of the incident light is referred to as an anti-Stokes line. In common with infrared absorption, Raman scattering indicates state of molecular vibration of the sample. However, what is observed in an infrared absorption spectrum is molecular vibration accompanied by change in dipole moment, whereas Raman scattering results from molecular vibration accompanied by change in polarizability. Therefore, these provide different types of information. In addition, infrared absorption analysis on a sample in the form of an aqueous solution is very difficult. In contrast thereto, use of Raman scattering yields an advantage that analysis on a sample dissolved in water becomes easy, because a Raman spectrum of water is weak.
Since Raman scattered light is faint, a laser capable of providing a monochromatic light beam having high intensity is used as a light source in measurement of Raman scattering As a spectroscope for measuring Raman scattering, a monochromator is used which has sufficient resolving power and acceptably low level of stray light in order to separate Raman scattered light from light resulting from Rayleigh scattering (Rayleigh scattered light). Such a monochromator includes a double monochromator using two diffraction gratings and a triple monochromator using three diffraction gratings. As a detector, there may be mentioned a type which uses a photomultiplier tube and performs wavelength (frequency) scanning by turning the diffraction gratings of the spectroscope, or a type which uses an optical multi-channel analyzer and measures a spectrum at a time. Further, Fourier-Raman spectrometry is known which uses an interference spectroscope as a spectroscope.
Besides Raman scattered lights light which is detected at a frequency shifted from the frequency of the incident light includes fluorescence. Fluorescence can be emitted from impurities contained in a sample or can be emitted from the sample per se. In the latter cases emission of fluorescence is unavoidable even if impurities are removed. Further, fluorescence generally has considerably high intensity as compared with those of Raman scattered light. This creates difficulty in detection of Raman scattered light.
As means for measuring Raman scattered light with emission of fluorescence suppressed, it may be conceived to use light in the infrared region, of which use is free from emission of fluorescent, as excitation light. For example, use of light emitted from a YAG laser and having a wavelength of 1.06 .mu.m may be conceived. However, use of infrared radiation as excitation light leads to disadvantageously weak Raman scattered light.
As other means for measuring a Raman spectrum with fluorescence separated, Japanese Unexamined Patent Publication No. 59693/1974 discloses a method which comprises irradiating a sample with a wavelength-modulated laser beam, separating light emitted from the sample by the irradiation with the laser beam into spectral components, detecting the spectral components, and selecting alternating current components from the detected signals. The method utilizes the fact that if a wavelength of an incident laser beam is shifted, a frequency of Raman scattered light is shifted owing to the shift of the wavelength of the incident laser beam, whereas a wavelength of a fluorescence is not shifted by the shift of the wavelength of the incident laser beam. A similar technique is also disclosed in each of Japanese Unexamined Patent Publication Nos. 80282/1976 and 39156/1978.
Each of Japanese Unexamined Patent Publication No. 60582/1974 and Japanese Examined Patent Publication No. 31893/1980 discloses a method which utilizes difference between degree of depolarization of Raman scattered light and degree of depolarization of fluorescence which are emitted from a sample to obtain a Raman spectrum free from influence of fluorescence. Japanese Examined Patent Publication No. 11511/1976 discloses a method which utilizes difference in lifetime between Raman scattered light and fluorescent to separate the Raman scattered light from the fluorescent.
Such spectrometric methods using a spectrophotometer are already established techniques and capable of carrying out measurement with high precision. In these methods, however, a monochromator for irradiation with monochromatic light is required, leading to an undesirably large-sized apparatus. Further, a light source cannot be separated from a detector. In consequence, size and state of a sample which can be placed in the spectrophotometer are restricted. Accordingly, it is impossible to pliably carry out spectrometry over samples in various state and under various conditions. Moreover, there is a problem that it is not easy to two-dimensionally measure a spectrometric spectrum of a sample such as an absorption spectrum or a reflection spectrum as a positional function of the sample.
On the other hand, in spectrometric methods using a laser as a light source, use of a monochromator is required because monochromatic light is obtained from a laser. However, a dye laser most widely used at present as a tunable laser has problems that a wavelength tunable range obtained by one dye is narrow, that since a diffraction grating, birefringent plate or the like is mechanically rotated to thereby effect wavelength selection, it is difficult to effect wavelength tuning at a high speed, and that in order to increase reproducibility, a wavelength can be swept only in one direction. Accordingly, a dye laser is not suitable for usual spectrometry such as measurement of absorption spectrum of a sample.
If an optical parametric oscillator (OPO) is utilized as a broad range tunable laser, a broad wavelength tunable range can be obtained. However, wavelength sweeping rate is low and reproducibility of wavelength is poor. Further, position and direction of an emitted light beam slightly vary depending upon its wavelength, and accordingly, its optical axis is unstable.
Spectroscopes using infrared semiconductor lasers have a disadvantage that since a wavelength tunable range obtained by one infrared semiconductor laser is narrow, use of a plurality of infrared semiconductor lasers and successive replacement thereof are required in order to effect measurement over a broad wavelength range.
With respect to a pulsed dye laser, a method for electrically sweeping an oscillation wavelength has been proposed which comprises placing a CaMoO.sub.4 crystal in a dye solution as a laser medium and applying an acoustic wave to the CaMoO.sub.4 crystal to provide a resonator so constructed as to resonate a component of a light beam which interacts with the acoustic wave, thereby tuning an oscillation wavelength of a laser (see Applied Physics Letters, Vol. 19, No. 8. pp. 269-271). However, the sweeping method has problems that a wavelength tunable range is narrow, that a complicated arrangement is required for integration of the crystal with the dye, that a specific crystal, i.e., a CaMoO.sub.4 crystal is required, and that it is difficult to separate the light beam component interacting the acoustic wave from light beam components non-interactive with the acoustic wave because difference therebetween resides in rotatory polarization.
Further, in measurement of a Raman spectrum, as described above, a double monochromator, triple monochromator or the like is used as a spectroscope to separate faint Raman scattered light from Rayleigh scattered light, thereby attaining measurement with high resolving power. However, brightness of a spectroscope is incompatible with resolving power of the spectroscope. Accordingly, if high resolving power is required, brightness is sacrificed, resulting in measurement over a prolonged period of time. In addition,, a spectroscope occupies a large space, and use thereof involves complicated procedure including settings of a slit width, scanning rate and time constant, and wavelength calibration.
In Raman spectroscopic analysis, it is important to eliminate influence of fluorescent. However, any of the above-described conventional means for separating Raman scattered light from fluorescence uses a spectroscope as spectroscopic means. Accordingly, Raman spectroscopic analysis has the same problem.